1. Field of the Invention
The present invention relates to the removal of substances contained in gases, such as gases flowing at low pressure. In particular, the present invention concerns a method and an apparatus for removing unreacted reactants and vapor phase precursors present in gases removed from vapor phase reactors.
2. Description of the Related Art
In the atomic layer deposition method (ALD), the substrate is typically located in a reaction space, wherein it is subjected to alternately repeated surface reactions of at least two different reactants. Commercially available technology is supplied by ASM Microchemistry Oy, Espoo Finland under the trademark ALCVD™. According to the method, the reactants are admitted repetitively and alternately one reactant at a time from its own source in the form of vapor-phase pulses in the reaction space. Here, the vapor-phase reactants are allowed to react with the substrate surface for the purpose of forming a solid-state thin film on the substrate, particularly for use in the semiconductor arts.
While the method is most appropriately suited for producing so-called compound thin films, using as the reactants starting materials or precursors that contain component elements of the desired compound thin-film, it may also be applied to growing elemental thin films. Of compound films typically used in the art, reference can be made to ZnS films employed in electroluminescent displays, whereby such films are grown on a glass substrate using zinc sulfide and hydrogen sulfide as the reactants in the growth process. Of elemental thin films, reference can be made to silicon thin films.
An ALD apparatus comprises a reaction space into which the substrate can be placed, and at least two reactant sources from which the reactants used in the thin-film growth process can be fed in the form of vapor-phase pulses into the reaction space. The sources are connected to the reaction space via reactant inflow channels. Outflow channels (pumping lines) are attached to a pump and connected to the reaction space for removing the gaseous reaction products of the thin-film growth process, as well as the excess reactants in vapor phase.
The waste, i.e., the non-reacted reactants removed and discharged from the reaction space, is a serious problem for ALD processing. When it enters the pumping line and the pump, the waste gives rise to tedious cleaning and, in the worst case, the pump will rapidly be worn out.
Filtering of the gases and/or contacting of the gases with absorbents gives some help but both methods have been shown to be unsatisfactory in the long run. Building expensive heated pumping lines in order to move the waste though the pump does not help, because the problematic waste does not comprise superfluous amounts of separate precursors, such as water, titanium chloride or aluminum chloride, that can easily be pumped as separate materials. The problem arises when the materials are reacting, forming by-products having a lower vapor pressure, inside the pumping line. The problem is especially relevant when the reactants react with each other at temperatures lower than the intended process temperature, causing improper reactions. At those temperatures, oxychlorides might form in exhaust lines as a by-product of exemplary metal oxide deposition processes using metal chlorides as one of the ALD precursors. These by-products form a high volume powder. Typically this kind of reaction happens inside the pumping line between the reaction zone and the colder parts of the pumping line. Another problem occurs when precursors with a high vapor pressure at room temperature reach the pump sequentially at temperatures suitable for film growth. This might lead to a film material build-up on the surfaces of the pump. The material build-up can be very abrasive. This is a specific problem with heated pumping lines and hot dry pumps. This will cause the filling of tight tolerances and due to that the parts will contact each other and pump will crash. A third problem is the reactions between condensed portions of the previous reaction component and the vapor of the following pulse in the pumping line. This will cause CVD-type material growth and significant powder propagation.
As mentioned above, different solutions based on filtering and/or chemical treatment of the reaction waste have been tried for decades in process fore-lines, with more or less poor results. Formed by-products and powder tend to block the filters and due to the low process pressure the gas flow is too weak to keep the mesh of the filter open. The blocked filter will cause an additional pressure drop and therefore cause changes in the material flow from the source. Also, the process pressure and the speed of the gases will change. Attempts have been made to use cyclones and rotating peelers to remove the by-products from the mesh. By these means, some of the solid waste can be removed, but still the precursors with high vapor pressure will reach the pump and form by-products there.
Finnish Patent No. 84980 (Planar International Oy) discloses a system consisting of a condensation chamber, where the gas stream is slowed down and where a big part of the waste is condensed. Before entering the filter unit, extra water is injected into the filter housing to increase the by-products' particle size in order to prevent blockage of the filter mesh before the waste is removed by a rotating peeler system. Although this apparatus represents a clear improvement of the state of the art, it is still not completely satisfactory.